fix: DNS split-horizon in DDNS mode, service access filter, health check, verbosity persistence

- DNS (critical): add _configured_dns_params() that returns (primary_domain,
  split_horizon_zones) from config_manager so all apply_all_dns_rules() callers
  pass the correct primary zone (e.g. 'pic.ngo') and split-horizon list
  (e.g. ['pic1.pic.ngo']) instead of the FQDN as the primary — fixes
  DNS_PROBE_FINISHED_BAD_CONFIG for all external domains when on VPN

- firewall_manager: add split_horizon_zones param to apply_all_dns_rules()
  and forward it to generate_corefile()

- Peers: filter service_access list to installed services only; peers.py
  derives valid services from config_manager.get_installed_services() with
  the email→mail ID mapping; Peers.jsx fetches from /api/store/installed
  and filters the checkboxes and defaults accordingly

- Health check: fix file_manager→'files' ID mapping so files service health
  is checked when installed (was silently skipped due to 'file' vs 'files')

- Verbosity persistence: move log_levels.json from non-mounted
  /app/api/config/ to CONFIG_DIR (/app/config/) which maps to config/api/
  on the host; both load (managers.py) and save (routes/services.py) updated

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/managers.py

@@ -121,9 +121,9 @@ _service_log_configs = {
 for _svc, _cfg in _service_log_configs.items():
     log_manager.add_service_logger(_svc, _cfg)
 
-# Apply any persisted log level overrides
+# Apply any persisted log level overrides (stored in the mounted config volume)
 import json as _json
-_levels_file = os.path.join(os.path.dirname(__file__), 'config', 'log_levels.json')
+_levels_file = os.path.join(CONFIG_DIR, 'log_levels.json')
 if os.path.exists(_levels_file):
     try:
         with open(_levels_file) as _lf: